Process for producing micropillar structure

ABSTRACT

A solution having a polymer dissolved in a hydrophobic organic solvent is cast on a substrate, said organic solvent is evaporated in a moist atmosphere to condense moisture contained in an atmosphere prevailing on a surface of said cast solution into micro-droplets, said micro-droplets are dispersed on the surface of said cast solution or in said cast solution into a close-packed structure, said micro-droplets, condensed and dispersed on the surface of said cast solution or in said cast solution, are evaporated to obtain a porous honeycomb structure with said droplets used as casts, and said porous honeycomb structure is at least bisected by peeling in a thickness direction, thereby obtaining honeycomb structures wherein micro-pillars or anisotropic micro-pillars are regularly formed and arranged by said bisection on the peeled sections.

ART FIELD

The present invention relates generally to a micro-pillar structurehaving quite a unique surface state, and particularly to a porousthin-film structure wherein ultramicro-pillars are regularly arranged ona substrate surface at a given spacing and its preparation process. Moreparticularly, the invention concerns a porous thin-film structure havinga water-repellent or hydrophilic surface, wherein ultramicro-pillars areregularly arranged on a substrate surface. The inventive structures,because of their own surface shape specificity, are all expected to haveever better advantages over structures that are free of micro-pillars orhave irregular pillars. All things considered, the invention pertains toa micro-pillar structure that is applicable or usable not only as cellculture engineering materials and medical scaffolding materials but alsoas semiconductors, recording materials, screens, separators, ionexchange membranes, battery separator materials, displays, opticalmaterials, optical guides, acoustic equipment materials or the like invarious technical fields.

BACKGROUND OF THE INVENTION

Mainly in the field of biochemistry, substrates with micropores or thelike regularly formed and arranged on their surfaces have recently beenincessantly proposed and made public as scaffolding materials in cellengineering and culture engineering in scientific literature andarticles.

Applications of each of the substrates proposed so far in the art tovarious art fields, to say nothing of medical fields, are now understudy. For instance, their applications to a diversity of fieldsincluding semiconductors, low-dielectric-constant materials, scatterlayers for electronic displays, magnetic recording material, photoniccrystals and cell culture substrates are now under study. Thus, suchsubstrates attract attention as one of promising materials.

However, attempts to prepare structures with such micropoores regularlyarranged thereon by means of ordinary micromachining processes runacross such problems as mentioned below, and it is very difficult toachieve them; such micromachining processes are still far away frompractically proper means. For instance, the micro-machining processesinclude lithography and laser machining. With these processes, however,there are some limits to the materials to be machined. In addition, analmost unlimitedly extraordinary number of micropores must be formedwith regularity in mind, and so by very troublesome operations. It isnot hard to imagine that micromachining requires a lot of steps and muchtime even though it is carried out by those pretty skilled in the art.As a matter of course, micromachining costs much.

Apart from this, there is known a so-called phase separation process bywhich micropatterns are formed. However, the resulting surface stateinvolves a reproducibility problem; only an inhomogeneous pattern isobtainable. Thus, that phase separation process is still less thansatisfactory for the formation of micropatterns having specificregularity.

Anyway, all the prior surface processing methods relying uponmicropatterning technology require ever higher levels of techniques, andso have a lot of problems such as the inability to achieve massproduction, and unavoidably increased costs.

By the way, some recent literature has reported that regularmicropatterns are relatively easily formed by casting a dilute polymersolution on a solid substrate. One typical process has been proposed bya researcher group including the present inventors (see non-patentpublication 1). According to this process, a dilute polymer solution iscast, and the solvent is evaporated thereby forming a dot (pillar,protuberance, or projection) pattern of microstructure in the polymer.Even that process is unsatisfactory, because it is still difficult toobtain any microdot pattern with such regularity as to control a dotarray.

It has also been proposed to form a porous film having a micro-honeycombpattern as a microstructure (non-patent publications 2 and 3). In thisprocess wherein a special polymer having a moiety of strongself-aggregation force and a flexibility-developing moiety incombination is used, that polymer is dissolved in a hydrophobic organicsolution, and the solution is then cast thereby forming said pattern.

The group including the present inventors has made an intensive study ofthis process as well, and succeeded in preparing a microstructure havinga unique honeycomb structure by choice of a specific polymer. Theresults have been reported in articles (non-patent publications 4 and5).

That is, the inventors have succeeded in the formation of a porous thinfilm having a honeycomb pattern structure by using as the constituent ofsaid polymer an amphiphilic polymer comprising a hydrophilic acrylamidepolymer as a main chain and having a dodecyl group as a hydrophobic sidechain and a lactose or carboxyl group as a hydrophilic side chain or anionic complex of an anionic polysaccharide such as heparin or dextransulfate with a quaternary long-chain alkyl-ammonium salt.

The inventors have also found that porous honeycomb structure filmsprepared from various biodegradable polymers provide an especiallypromising material for cell culture substrates, and filed a patentapplication for them (patent publication 1).

The preparation process proposed by the inventors in that patentapplication involves an extremely simplified operation wherein a poroushoneycomb structure film is obtainable by blowing a high-humidity aironto a cast film of a hydrophobic organic solution having a controlledconcentration or the cast film is placed in a high-humidity atmosphere,and so is favorable in terms of preparation cost.

In that case, the pore diameter of the porous film can be controlled inthe range of 0.1 to 100 μm by changing the diameter of water dropletsacting as pore casts. Thus, the proposal by the inventors is of greatoriginality and excellence.

Non-Patent Publication 1

-   Chemistry Letters, 821, 1996

Non-Patent Publication 2

-   Science 283, 373, 1999

Non-Patent Publication 3

-   Nature 369, 387, 1994

Non-Patent Publication 4

-   Thin Solid Films 327, 829, 1998

Non-Patent Publication 5

-   Molecular Cryst. Liq. Cryst. 322, 305, 1998

Patent Publication 1

-   JP(A) 200-1157574

The studies and proposals mentioned above underlie the presentinvention. One object of the invention is to achieve a further extensionof them so that a structure that has not just a honeycomb texture butalso with micro-pillars formed on its surface can be obtained by meansof an extremely simplified process.

Another object of the invention is to provide, through a very simplifiedprocess, a micro-pillar structure with anisotropy imparted thereto, amicro-pillar structure enriched in water repellency, and a micro-pillarstructure enriched in hydrophilicity.

DISCLOSURE OF THE INVENTION

As a result of intensive studies of the technology mentioned above and afurther extension thereof, the inventors have now succeeded in obtaininga structure that has quite an unheard-of surface properties, i.e., anregular array of micro-pillars. That is, the inventors have succeeded inthe preparation of a structure having absolutely unique micro-pillars bystarting from a solution of a polymer in a hydrophobic organic solventto obtain a porous thin film of honeycomb structure as an intermediate,and bisecting the intermediate by peeling in its thickness direction.

Having been made on the basis of this success and findings obtainedalong the way, the present invention is embodied as follows.

(1) A micro-pillar structure, characterized in that micro-pillars areregularly arranged on a substrate surface.

(2) A micro-pillar structure having a water-repellent surface,characterized in that micro-pillars are regularly arranged on asubstrate surface, and have water repellency.

(3) A micro-pillar structure having a hydrophilic surface, characterizedin that micro-pillars are regularly arranged on a substrate surface, andare made hydrophilic by a hydrophilicity-imparting treatment.

(4) A micro-pillar structure according to any one of (1) to (3) above,wherein said substrate and said micro-pillars comprise a polymer and, ifrequired, includes a modifier.

(5) A micro-pillar structure according to any one of (1) to (4) above,wherein said substrate and said micro-pillars use as a precursor aporous honeycomb structure comprising a polymer and, if required,including a modifier, and said structure is obtained by bisecting saidprecursor by peeling in a thickness direction.

(6) A micro-pillar structure according to (5) above, wherein saidprecursor or the substrate obtained from said precursor is in athin-film form.

(7) A micro-pillar structure according to any one of (1) to (5) above,wherein said micro-pillars are arranged at a length of 0.1 to 50 μm, atip length of 0.01 to 20 μm and a spacing of 0.1 to 100 μm.

(8) A micro-pillar structure according to claim 4 or 5, wherein saidpolymer includes a hydrophobic or biodegradable polymer, and includes anamphiphilic polymer.

(9) A micro-pillar structure according to (8) above, wherein saidpolymer comprises 50 to 99% of said hydrophobic polymer and/or saidbiodegradable polymer with the rest being said amphiphilic polymer.

(10) A micro-pillar structure according to (8) or (9) above, wherein apolyester, a poly(meth)acrylate, a polycarbonate or a polystyrene isused as said hydrophobic or biodegradable polymer.

(11) A micro-pillar structure according to any one of (1) to (10) above,characterized in that a solution having a polymer dissolved in ahydrophobic organic solvent is cast on a substrate, said organic solventis evaporated in a moist atmosphere to condense moisture contained in anatmosphere prevailing on a surface of said cast solution intomicro-droplets, said micro-droplets are dispersed on the surface of saidcast solution or in said cast solution into a close-packed structure,said micro-droplets, condensed and dispersed on the surface of said castsolution or in said cast solution, are evaporated to obtain a poroushoneycomb structure with said droplets used as casts, and said poroushoneycomb structure is at least bisected by peeling in a thicknessdirection, thereby obtaining honeycomb structures wherein micro-pillarsare regularly formed and arranged by said bisection on peeled sections.

(12) A micro-pillar structure according to any one of (1) to (11) above,characterized in that said micro-pillars are oriented in any directionexcept for a vertical direction and set with anisotropy.

(13) A micro-pillar structure according to (12) above, characterized inthat said anisotropic micro-pillars are obtained by a peeling treatmentincluding transverse shearing stress in such a way that when the poroushoneycomb structure that is a micro-pillar precursor is sectioned bypeeling in the thickness direction, the resulting micro-pillars areoriented in any direction except for the vertical direction.

(14) A micro-pillar structure according to (3) above, wherein saidhydrophilicity-imparting treatment is any one or a combination of achemical modification treatment, an ozone oxidization treatment and analkali treatment.

(15) A process for preparing a micro-pillar structure, characterized inthat a solution having a polymer dissolved in a hydrophobic organicsolvent is cast on a substrate, said organic solvent is evaporated in amoist atmosphere to condense moisture contained in an atmosphereprevailing on a surface of said cast solution into micro-droplets, saidmicro-droplets are dispersed on the surface of said cast solution or insaid cast solution into a close-packed structure, said micro-droplets,condensed and dispersed on the surface of said cast solution or in saidcast solution, are evaporated to obtain a porous honeycomb structurewith said droplets used as casts, and said porous honeycomb structure isat least bisected by peeling in a thickness direction, thereby obtaininghoneycomb structures wherein micro-pillars are regularly formed andarranged by said bisection on peeled sections.

(16) A process for preparing a micro-pillar structure according to (15)above, characterized in that said polymer is composed of a hydrophobicor biodegradable polymer and an amphiphilic polymer and, if required, amodifier is incorporated therein.

(17) A process for preparing a micro-pillar structure according to (16)above, wherein said polymer comprises 50 to 99% of said hydrophobicpolymer or said biodegradable polymer with the rest being saidamphiphilic polymer.

(18) A process for preparing a micro-pillar structure according to (16)or (17) above, characterized in that said hydrophobic or biodegradablepolymer comprises a polymer having a polyester, a poly(meth)acrylate, apolycarbonate or a polystyrene as a basic skeleton.

(19) A process for preparing a micro-pillar structure according to (15)above, wherein said moist atmosphere is adjusted to a relative humidityof 50 to 95%.

(20) A process for preparing a micro-pillar structure according to (15)or (19) above, characterized in that said atmosphere is an ordinary airatmosphere.

(21) A process for preparing a micro-pillar structure according to (15)above, characterized in that operation for evaporation of said organicsolvent in said moist atmosphere is carried out by blowing an atmospherehaving a high humidity onto an evaporation interface of said organicsolvent.

(22) A process for preparing a micro-pillar structure according to (15)above, characterized in that peeling operation is carried out by use ofan adhesive tape.

(23) A process for preparing a micro-pillar structure according to (15)above, characterized in that peeling operation is carried out bydissolution of the polymer.

(24) A process for preparing a micro-pillar structure according to (15)above, characterized in that peeling operation is carried out byultrasonic irradiation.

(25) A process for preparing a micro-pillar structure according to anyone of (15) to (24) above, characterized in that said micro-pillars arearranged at a length of 0.1 to 50 μm, a tip length of 0.01 to 20 μm anda spacing of 0.1 to 100 μm.

(26) A process for preparing a micro-pillar structure according to anyone of (15) to (25) above, wherein said micro-pillars are oriented inany direction except for a vertical direction and set with anisotropy.

(27) A process for preparing a micro-pillar structure according to (26)above, characterized in that said anisotropic micro-pillars are obtainedby a peeling treatment with transverse shearing stress in such a waythat when the porous honeycomb structure that is a micro-pillarprecursor is sectioned by peeling in the thickness direction, theresulting micro-pillars are oriented in any direction except for thevertical direction.

ADVANTAGES OF THE INVENTION

The present invention provides quite an unheard-of material by casting adilute polymer solution onto a solid substrate with water vapor used ascasts to obtain a thin film of a honeycomb structure having an orderlymicro-pattern, and sectioning the thin film by peeling in its thicknessdirection thereby obtaining thin-film micro-pillar structures withmicro-pillars regularly arranged and formed on the sectioned thin-filmsections. This novel material, because of having striking surfaceproperties due to micro-pillars regularly arranged on its surface, couldhereafter find applications, with a great deal of advantages, in thefollowing various fields: as chemical valves, DNA chips, protein chipsand cyto-diagnosis chips, for cell culture engineering, as medicalscaffolding materials, semiconductors, recording materials, separators,ion exchange membranes, battery separator materials, optical materialsfor displays and light guides, catalyst carriers, cell culturesubstrates and anisotropic solid, electrically conductive materials, formicro-passageways, etc. Further, the material of the invention couldprovide, with a good deal of advantages, a surface well suitable as abiochip surface for controlling material flows in a constant directionand a low friction resistance surface for diminishing air or waterresistance in a certain direction alone. However, such advantages arenothing more than exemplification; the micro-pillar structure of thematerial of the invention could contribute much to accelerated contacteffects or operations for separation of liquids by evaporation anddrying in various reaction operations by gas-liquid contact reactions orliquid-liquid contact reactions.

The material of the invention also could possibly be used for specificflow passages, for instance, because pores formed on its surfacecooperate with micro-pillars connected thereto to cause a liquid flowingon the surface that faces away the micro-pillars to be discharged to anexternal space through the combined pores and micro-pillars. That is,the material of the invention could be used not only as a medicalmaterial used for designing artificial blood vessels, artificialkidneys, etc., but also as kinetic materials for microrobots,micro-biorobots or the like, because the micro-pillars can be controlledto put them in any desired artificial ciliary movement. Further, thematerial of the invention could possibly be used for designing filterswith reduced pressure losses, because the above arrangement enablesliquids to be effectively discharged out of filter operations.Furthermore, the novel material of the invention, because of itsfree-form feature, could be used in various forms as well as in variousapplications. Thus, the material of the invention could hereafter beapplied and used as a new material in microtechnology that will be evermore advanced with progresses in nanotechnology, with a great deal ofadvantages resulting from the specificity of the surface shape.

In particular, the aforesaid micro-pillar structure having a hydrophobicsurface would be much more often used in technical fields where waterrepellent surfaces are now in stronger need than ever before. Suchtrends and applications are reviewed in a number of publications, andnumerous aspects of various applications are reviewed and and mentionedthere (for example, 1. “State-of-the-art trends in high water-repellencytechnology—from ultra-water repellent materials to the latestapplications”, Toray Research Center, 2001, and 2. “MaterialIntegration”, Vol. 14, No. 10, 2001). Still, with all processing methodsso far available in the art, there are some limits to the materials tobe processed, and much time and added costs are taken for processingsteps. Aspect (2) of the invention and its related sub-aspects canprovide a new material enough to meet such needs by a simple formingmeans. From now on, the material of the invention is expected to enjoywide uses in diverse fields and make a great contribution todevelopments.

In some fields where the aforesaid water repellency is needed, there isan increasing need for an affinity for water. For designing filters forfiltering purposes, for instance, it is required to improve theefficiency of separation of water from the surface of a filter materialand, hence, keep the material hydrophilic. Aspect (3) of the inventionand its related sub-aspects concerning the hydrophilic surface canprovide a new material that has a surface rich in hydrophilicity so asto meet that.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is illustrative of an SEM image of the porous honeycomb structure(film) obtained on a laboratory dish as described in Example 1, and anSEM image of its surface.

FIG. 2 is illustrative of an SEM image (top) of the porous honeycombstructure described in Example 1, and an SEM image (bottom) of itsthree-dimensional structure.

FIG. 3 is illustrative of an SEM image of the micro-pillar structure ofExample 1 obtained after sectioning by peeling, as taken from an obliquedirection.

FIG. 4 is illustrative of SEM images of the porous honeycomb structures(films) made of various polymers set forth in Example 2.

FIG. 5 is illustrative of SEM images of the micro-pillar structures thatwere prepared by peeling tapes off the porous honeycomb structures(films) of Example 2.

FIG. 6 is illustrative of an SEM image of the micro-pillar structure ofExample 3.

FIG. 7 is illustrative of SEM images of the micro-pillar structuresobtained by the dissolution of the polymer in Example 4.

FIG. 8 is illustrative of an SEM image of the micro-pillar structureobtained by ultrasonic irradiation in Example 5.

FIG. 9 is illustrative of SEM images of the anisotropic micro-pillarstructure of Example 6.

FIG. 10( a) is illustrative of an SEM image of the anisotropicmicro-pillar structure of Example 7, as taken from right above, andFIGS. 10( b) and 10(c) are illustrative of its SEM images as taken froma 55° oblique direction.

BEST MODE OF CARRYING OUT THE INVENTION

As already explained, the micro-pillar structure of the invention isprepared through two steps, one for obtaining a porous honeycombstructure and another for sectioning it off by peeling. Such a series ofoperations will now be summarized below.

At the first step, a solution of a hydrophobic or biodegradable polymerand an amphiphilic polymer dissolved in a hydrophobic organic solvent isprovided, and then cast on a substrate. Subsequently, the organicsolvent is slowly evaporated in an air having a relative humidity of 50%or higher or, alternatively, it is evaporated by blowing a high-humidityatmospheric gas to the surface of the cast solution, so that moisture iscondensed on the surface of the cast solution by the latent heat ofevaporation into micro-droplets that are dispersed on the surface of thesolution or in the solution into a close-packed structure. Finally, themicro-droplets, condensed and dispersed on the surface of the solutionor in the solution, are evaporated to obtain a porous honeycombstructure with the droplets used as templates. At the second step, anadhesive tape is applied over the resulting honeycomb structure, andthen peeled off by pulling or other peeling means to bisect thehoneycomb structure, thereby obtaining structures wherein micro-pillarsformed by fracturing of the honeycomb structure are regularly arrangedon the bisected sections.

More specifically, in the process of obtaining the porous honeycombstructure with the droplets used as templates at the first step, themoisture is condensed on the surface of the cast solution intomicro-droplets dispersed on the surface of the cast solution or in thecast solution into a close-packed structure, and the droplets are thenevaporated to obtain a structure with closely packed micropores of 0.1to 100 μm in diameter, i.e., a thin-film form of honeycomb structure. Aspreviously described, that process itself has been proposed on the basisof a specific polymer, and filed in the form of a patent application.However, it has now been found that the honeycomb structure (FIG. 1)formed by the above process has a structure wherein, with themicro-droplets used as casts, one pore is supported by six posts thatare constricted in the middle (FIG. 2); such findings underlie theinvention.

That is, the porous honeycomb structure is sectioned off by peeling,thereby obtaining a state or structure wherein micro-pillars are quiteregularly arranged and formed on the surfaces sectioned by peeling.

In the invention, the first process is linked to the second processwherein the honeycomb structure is broken away by peeling operation,thereby obtaining a texture with micro-pillars of given length formed ata given spacing.

It is here noted that FIG. 1 is illustrative of a porous honeycombstructure, as formed in a laboratory dish and as scaled up to twoscales, and FIG. 2 is illustrative of an image thereof further scaled upby an electron microscope (SEM), and illustrates in schematic athree-dimension texture thereof.

For the hydrophobic or biodegradable polymer employed herein, forinstance, it is preferable in view of solubility in the organic solventto use polylactic acid; lactic acid-glycolic acid copolymers;polyhydroxybutryic acid, polycaprolactone; biodegradable aliphaticpolyesters such as polyethylene adipate and polybutylene adipate;aliphatic polycarbonates such as polybutylene carbonate and polyethylenecarbonate; polystyrenes; polysulfones; polymethylhexadecylsiloxanes;polyvinylcarbazoles; poly-tetrahydrofulfuryl methacrylates; polybutylmethacrylates; polymethyl methacrylates; and polycarbonates.

For the amphiphilic polymer employed herein on which no particularlimitation is placed, it is preferable to use amphiphilic polymershaving a polyethylene glycol-polypropylene glycol block copolymer or anacrylamide polymer as a main chain skeleton and containing a dodecylgroup as a hydrophobic side chain and a lactose or carboxyl group as ahydrophilic side chain, or amphiphilic polymers having as a hydrophilicgroup a water-soluble protein, for instance, an ion complex of ananionic polymer such as heparin or nucleic acids, e.g., dextran sulfateDNA or RNA with a long-chain alkyl ammonium salt, gelatin, collagen, andalbumin, because they are easily available.

For preparing the porous-honeycomb-structure thin film that provides themicro-pillar structure of the invention, it is imperative to formmicro-droplets on the polymer solution, and so the organic solvent usedmust be a water-insolbule one. It is also necessary that each polymerused be soluble in the organic solvent. Yet another requirement for theorganic solvent is that it evaporates off readily, and causes moistureto condense readily by the latent heat of evaporation.

While the organic solvent is not particularly limited insofar as itpossesses such properties, it is understood that it is desired to useeasily available, toxicity-free organic solvents. Specific mention ismade of, for instance, halogen-based organic solvents such aschloroform, dichloromethane and dichloroethane; aromatic hydrocarbonssuch as benzene, toluene and xylene; esters such as ethyl acetate andbutyl acetate; water-insoluble ketones such as methyl isobutyl ketone;and carbon disulfide. Those organic solvents may be used alone or in theform of a mixed solvent comprising a combination of two or more.

The combined concentration of both the hydrophobic or biodegradablepolymer and the amphiphilic polymer, all soluble in the organicsolvent(s), is in the range of preferably 0.01 to 20 wt %, and morepreferably 0.1 to 10 wt %. Polymer concentrations of lower than 0.01 wt% are not desired because the ensuing thin film has insufficient dynamicstrength. Too high polymer concentrations of more than 20 wt % fail togive any good enough honeycomb structure. The composition ratio of thehydrophobic or biodegradable polymer to the amphiphilic polymer shouldbe in the range of 99:1 to 50:50 (wt/wt). The amphiphilic polymer ratiosof less than 1 and more than 50 are not preferable because in the formercase no uniform porous structure is obtainable and in the latter casethe resulting porous thin film lacks stability, especially dynamicstability.

In the invention, the solution of those polymers in the organic solventis cast on the substrate to prepare a porous thin film. For thatsubstrate, it is preferable to avoid use of substrate materials that aremutually dissoluble in the solution of the polymers dissolved in theorganic solvent or undergo corrosion and reaction therewith or rely onchemically stable substrate materials. For instance, mention is made ofinorganic materials such as glasses, metals and silicon wafers; andpolymers excellent in resistance to organic solvents, such aspolypropylene, polyethylene, polyether ketone and fluoro-resin. Whileall the above substrate materials have a solid surface, it is understoodthat the invention is not always limited to them. In other words, it isacceptable to make use of liquids such as water, liquid paraffin andliquid polyether. After the starting polymer solution is spread on thatliquid, similar operation is carried out or, specifically, the organicsolvent is evaporated in a high-humidity atmosphere to obtain a thinfilm having a porous honeycomb structure with droplets used as casts.Thereafter, similar peeling operation is performed to formmicro-pillars. Use of the liquid for the substrate is rather preferableto prepare thinner films because the thickness of the polymer solutionspread on the substrate can be controlled by surface tension.

The structure can be easily removed by itself from any of the substrateswhile taking advantage of the feature, i.e., self-supportability, of thethin film of the porous honeycomb structure.

In the invention, a possible mechanism of forming the porous honeycombstructure could be as follows. As the hydrophobic organic solvent isevaporated, it gets rid of latent heat, lowering the temperature of thesurface of the cast thin film to condense moisture, thereby aggregatingand depositing micro-droplets of water on the surface of the polymersolution. The hydrophilic moiety in the polymer solution then works todecrease the surface tension between water and the hydrophobic organicsolvent, so that micro-particles of water can be stabilized againstcoming together into an aggregate. As the solvent is further evaporated,it allows hexagonal droplets to be arranged in close-packed form and,eventually, as the water is evaporated off, it allows the polymer to beused as arranged in orderly honeycomb form. Therefore, the environmentin which the thin film is to be prepared should preferably have arelative humidity in the range of 50 to 95%. A relative humidity of lessthan 50% is not preferable because the condensation of water on the castthin film becomes insufficient, and a relative humidity of more than 95%is again not preferable because environment control becomes difficult.

As can be appreciated from the foregoing, the “porous honeycombstructure” of the invention has a double-layer texture in which twofilms are put one upon another (see FIG. 2) with micropores regularlyarranged between them, wherein each micropore is supported by six poststhat are constricted in the middle. A possible reason for the formationof such a complicated and orderly texture could be that atmosphericmoisture is condensed by the latent heat of evaporation of the solventon the surface of the cast polymer solution and water droplets arehexagonally close-packed, causing the polymer to be precipitated in aspace except for the water droplets.

The pore diameter and thickness of the porous thin film may becontrolled by varying the concentration of the solution to be cast, thetype of the solvent, the amount of the solution, the prevailingatmosphere or the flow rate, temperature and humidity of the air to beblown, i.e., controlling the evaporation speed of the solvent and thecondensation speed in a suitable combination, whereby the growth ofwater droplets that provide the casts for pore diameter and theevaporation rate of the solvent are controllable. The pore diameter andthickness of the thus obtained porous thin film that is the precursor ofthe micro-pillar structure are in the range of 0.1 to 100 μm.

The high-humidity air to be blown onto the film may have a relativehumidity such that atmospheric moisture can be condensed on the surfaceof the film, i.e., a relative humidity of 20 to 100% although it varieswith temperature. In the invention, not only is air usable butrelatively inert gases such as nitrogen and argon may be used as well.

The flow rate of the high-humidity air to be blown onto the film may besuch that the atmospheric air is condensed on the surface of the filmand the solvent used for casting is evaporated.

The temperature of the atmosphere for blowing the high-humidity air maybe such that the solvent used for casting is evaporated.

By way of illustration but not by way of limitation, the peelingoperation for the surface of the porous thin film is typically carriedout by means of peeling off an adhesive tape applied over the surface ofthe thin film. The peeling operation by the adhesive tape is capable offorming a micro-pillar structure even on a curved substrate, because asimilar micro-pillar structure can be formed on either a substrate sideor a tape side. Some other peeling operations are also usable, includingultrasonic irradiation, or dissolution of polymer. It is here noted thatthe micro-pillar structure formed on the substrate side can be easilypeeled off as a self-supporting thin film.

The porous-honeycomb-structure thin film provides the precursor of themicro-pillar structure, and so the spacing between the micro-pillars isin the range of about 0.1 to 100 μm depending on the pore diameter ofthe porous thin film. The height and tip size of the micro-pillarstructure are in the ranges of about 0.1 to 100 μm and 0.01 to 20 μm,respectively, depending on the thickness of the porous thin film, thepeeling operation applied and the material used.

It is here noted that the “micro-pillar structure” used herein refers toa structure in which a plurality of pillars of substantially constantheight are regularly arranged at a substantially constant spacing.Sectional shape of each pillar is not exclusively any of circular, oval,hexagonal, rectangular, square or other shapes.

The micro-pillar structure of the invention, because of havingmicro-pillars formed thereon, has much better properties than structuresfree from micro-pillars or having irregular micro-pillars, such asdecreased surface resistance and much improved water repellency. To addto this, the micro-pillar structure of the invention could haveapplications as a culture substrate in cell culture technology toimprove the rate of deposition of cells, producing excellent actions oncell growth and differentiation, etc. As a matter of course, themicro-pillar structure of the invention is quite an unheard-of materialin view of not only its surface properties but also its generally uniquemicro-texture, and so is of great significance. The inventors make surethat novel structure could hereafter have great impacts on materialdesigning in various fields, and give rise to far better advantagescontributing much to developments in the industry.

EXAMPLES

The present invention is now explained with reference to the drawingsand examples. However, the examples are given as an aid to a betterunderstanding of the invention and the invention is not limited theretowhatsoever.

Example 1

Four (4) ml of a chloroform solution (having a polymer concentration of4 mg/l) in which a polystyrene having an average molecular weight of200,000 was mixed with an amphiphilic polyacrylamide (compound 1Capformally called dodecylacrylamide-ω-carboxyhexylacrylamide) at a weightratio of 10:1 were cast on a glass laboratory dish of 10 cm in diameter,and the chloroform solvent was evaporated with a high-humidity airhaving a relative humidity of 70% blown onto the solution in a flow rateof 2 l per minute to prepare a porous thin film having a honeycombpattern structure (FIGS. 1 and 2). An adhesive tape was then appliedover the surface of a film-like sample piece obtained in the laboratorydish, after which the tape was peeled off in a thickness direction toprepare a micro-pillar structure. Oblique observation under an electronmicroscope (hereinafter called SEM for short) indicated that structureswith micro-pillars, having quite high regularity, could be obtained onboth a tape side and a glass laboratory dish side (FIG. 3).

Structural Formula of Cap

Example 2

Six (6) ml of each of chloroform solutions (having a polymerconcentration of 4 mg/l) in which nine polymers, i.e., 1. polystyrene,2. polymethyl methacrylate, 3. polycarbonate, 4. polytetrahydrofurfurylmethacrylate, 5. poly(∈-caprolactone), 6. polylactic acid, 7.poly(glycolic acid-lactic acid) copolymer (having a composition rate of50:50), 8. polysulfone, and 9. polymethylhexadecylsiloxane were eachmixed with an amphiphilic polyacrylamide (compound 1Cap:dodecylacrylamide-ω-carboxyhexyl-acrylamide) at a weight ratio of 10:1were cast on a glass laboratory dish of 10 cm in diameter, and thechloroform solvent was evaporated with a high-humidity air having arelative humidity of 70% blown onto the solution in a flow rate of 3 lper minute to prepare a porous thin film having a honeycomb patternstructure in which ultramicro-pores all lined up regularly (FIG. 4). Asa result of similar peeling operation as in Example 1, a micro-pillarstructure could be prepared, in which posts lined up regularly abouteach micropore to support it ruptured so that ultramicro-pillars linedup regularly (FIG. 5).

Example 3

A chloroform solution (having a polymer concentration of 4 mg/l) inwhich a polytetrahydrofurfuryl methacrylate having an average molecularweight of 200,000 was mixed with an amphiphilic polyacrylamide at aweight ratio of 10:1 were cast in varied amounts of 2.5, 5, 7.5 and 10ml on a glass laboratory dish of 10 cm in diameter, and the chloroformsolvent was evaporated with a high-humidity air having a relativehumidity of 70% blown onto the solution in a flow rate of 2 l per minuteto prepare a porous thin film having a honeycomb pattern structure. Anadhesive tape was then applied over the surface of the film, after whichthe tape was peeled off in a thickness direction to prepare amicro-pillar structure. SEM observations indicated that structures withmicro-pillars having a varying pillar spacing could be prepareddepending on the amount of the solvent cast (FIG. 6).

Example 4

Eight (8) ml of chloroform solution (having a polymer concentration of 4mg/l) in which a polystyrene was mixed with an amphiphilicpolyacrylamide (compound 1Cap:dodecylacrylamide-ω-carboxyhexylacrylamide) at weight ratios of (a)10:1, (b) 10:2, (c) 10:2.5 and (d) 10:3 were cast on a glass laboratorydish of 10 cm in diameter, and the chloroform solvent was evaporatedwith a high-humidity air having a relative humidity of 70% blown ontothe solution in a flow rate of 3 l per minute to prepare porous thinfilms having a honeycomb pattern. Each porous thin film was dissolved in1-propanol by a 10-minute dipping, followed by washing with ethanol.After drying, SEM observations (FIG. 7) were performed. As a result, itwas found that each sample piece could be sectioned off by peeling,yielding a structure in which micro-pillars or micro-pillar patternslined up regularly oh the sectioned surfaces. For the purpose ofcomparison, a structure obtained by tape peeling operation was shown onthe right side. As a result, it was found that thedissolution-of-the-polymer operation, too, was an effective peelingmeans.

Example 5

Five (5) ml of a chloroform solution (having a polymer concentration of4 mg/l) in which a polystyrene was mixed with an amphiphilicpolyacrylamide (compound 1Cap:dodecylacrylamide-ω-carboxyhexylacrylamide) at a weight ratio of 10:1were cast on a glass laboratory dish of 10 cm in diameter, and thechloroform solvent was evaporated with a high-humidity air having arelative humidity of 70% blown onto the solution in a flow rate of 3 lper minute to prepare a porous thin film having a honeycomb pattern. Thethus obtained porous thin film was irradiated with ultrasonic waves (20KHz, 15 W) for 5 minutes. As a result, it was shown that the honeycombstructure formed with the water droplets used as casts was cut at postsand micro-pillars were popped out. Through SEM observations, ultrasonicirradiation was shown to be an effective peeling means (FIG. 8).

Example 6

Eight (8) ml of a chloroform solution (having a polymer concentration of4 mg/l) in which a polycarbonate having an average molecular weight of29,000 was mixed with an amphiphilic polyacrylamide (compound 1Capformally called dodecylacrylamide-ω-carboxyhexylacrylamide) at a weightratio of 10:1 were cast on a glass laboratory dish of 10 cm in diameter,and the chloroform solvent was evaporated with a high-humidity airhaving a relative humidity of 70% blown onto the solution in a flow rateof 2 l per minute to prepare a porous thin film having a honeycombpattern (FIG. 9( a)). An adhesive tape was then applied over the surfaceof the film, after which the adhesive tape was peeled off with theapplication of transverse shearing stress to prepare anisotropicmicro-pillar structures (FIGS. 9( b) and 9(c)). As a result of SEMoblique observations, it was found that anisotropic micro-pillarstructures having quite high regularity could be obtained on both a tapeside and a glass laboratory dish.

Example 7

Six (6) ml of a chloroform solution (having a polymer concentration of 4mg/l) in which a polystyrene having an average molecular weight of1,000,000 was mixed with an amphiphilic polyacrylamide at a weight ratioof 10:1 were cast on a glass laboratory dish of 10 cm in diameter, andthe chloroform solvent was evaporated with a high-humidity air having arelative humidity of 70% blown onto the solution in a flow rate of 3 lper minute to prepare a porous thin film having a honeycomb pattern(FIG. 10( a)). An adhesive tape was then applied over the surface of thefilm, after which the adhesive tape was peeled off with the applicationof transverse shearing stress to prepare anisotropic micro-pillarstructures (FIGS. 10( b) and 10(c)). As a result of SEM obliqueobservations, it was found that anisotropic micro-pillar structureshaving quite high regularity could be obtained on both a tape side and aglass laboratory dish.

Examples 8 to 13 are now given to show that the inventive micro-pillarstructure, prepared from the hydrophobic polymer, has a water-repellentsurface as compared with an ordinary film structure (hereinafter calleda plain film) or a honeycomb structure with no micro-pillars, preparedfrom the same material.

Example 8 and Comparative Examples 1, 2

Six (6) ml of a chloroform solution (having a polymer concentration of 4mg/l) in which a polystyrene having an average molecular weight of200,000 (Aldrich) was mixed with an amphiphilic polyacrylamide (compound1Cap: dodecylacrylamide-ω-carboxyhexylacrylamide) at a weight ratio of10:1 were cast on a glass laboratory dish of 10 cm in diameter, and thechloroform solvent was evaporated with a high-humidity air having arelative humidity of 80% blown onto the solution in a flow rate of 3 lper minute to prepare a porous thin film having a honeycomb pattern. Anadhesive tape was then applied over the surface of the film, after whichthe adhesive tape was peeled off to prepare anisotropic micro-pillarstructures. Ten (10) μl of distilled water were added drop-wise ontoeach micro-pillar structure, and in 30 seconds later, the static contactangle with water was measured. For the purpose of comparison andcontrol, the static contact angles with water of the surface of apolystyrene porous film and a plain film prepared using a chloroformsolution of the above polystyrene and Cap (at a weight ratio of 10:1)with a spin coater were measured. The static contact angles with water,as measured, are set out in Table 1.

Example 9 and Comparative Examples 3, 4

Example 1 was repeated using a polymethyl methacrylate having an averagemolecular weight of 350,000 (Aldrich) instead of the polystyrene toprepare a porous thin film having a honeycomb pattern structure, amicro-pillar structure and a plain film, and their static contact angleswith water were measured. The results are set out in Table 1.

Example 10(c) and Comparative Examples 5, 6

Example 1 was repeated using a polycarbonate having an average molecularweight of 29,000 instead of the polystyrene to prepare a porous thinfilm having a honeycomb pattern structure, a micro-pillar structure anda plain film, and their static contact angles with water were measured.The results are set out in Table 1.

Example 10(c) and Comparative Examples 5, 6

Example 1 was repeated using a polycarbonate having an average molecularweight of 29,000 instead of the polystyrene to prepare a porous thinfilm having a honeycomb pattern structure, a micro-pillar structure anda plain film, and their static contact angles with water were measured.The results are set out in Table 1.

Example 11 and Comparative Examples 7, 8

Example 1 was repeated using a polytetrahydro-furfuryl methacrylatehaving an average molecular weight of 240,000 instead of the polystyreneto prepare a porous thin film having a honeycomb pattern structure, amicro-pillar structure and a plain film, and their static contact angleswith water were measured. The results are set out in Table 1.

Example 12(e) and Comparative Examples 9, 10

Example 1 was repeated using a poly(∈-caprolactone) having aviscosity-average molecular weight of 40,000 (made by Wako Junyaku Co.,Ltd.) instead of the polystyrene to prepare a porous thin film having ahoneycomb pattern structure, a micro-pillar structure and a plain film,and their static contact angles with water were measured. The resultsare set out in Table 1.

Example 13(f) and Comparative Examples 11, 12

Example 1 was repeated using a poly(glycolic acid-lactic acid) copolymer(available from Aldrich with a composition ratio of 50:50) having aweight-average molecular weight of 40,000 to 75,000) instead of thepolystyrene to prepare a porous thin film having a honeycomb patternstructure, a micro-pillar structure and a plain film, and their staticcontact angles with water were measured. The results are set out inTable 1.

TABLE 1 Comparative Inventive Inventive/ Contact Angle Contact AngleComparative Polymer Plain Honeycomb Micro-Pillars 1/(1, 2) Polystyrene98° ± 3° 120° ± 2° 159° ± 3° 2/(3, 4) Polymethyl 96° ± 3° 120° ± 2° 158°± 4° Methacrylate 3/(5, 6) Polycarbonate 97° ± 3° 122° ± 3° 162° ± 3°4/(7, 8) Poly(tetrahy- 85° ± 3° 121° ± 2° 158° ± 2° drofurfuryl-methacrylate) 5/(9, 10) Poly (ε- 80° ± 3° 120° ± 2° 151° ± 1°caprolactone) 6/(11, 12) Poly(glycolic 91° ± 1° 127° ± 3° 164° ± 2°acid-lactic acid) 50:50

Example 14 is now given to show that a micro-pillar structure havinggood wettability by water and rich hydrophilicity can be prepared by theapplication of hydrophilicity-imparting means to the surfaces ofmicro-pillars comprising polymers.

Example 14 Surface Modification by Titanium Alkoxide

A polylactic acid (PLLA) and Cap were mixed together at a ratio of 1:1to prepare 2.0 g/l of a chloroform solution. This solution was cast on alaboratory dish of 9 cm in diameter and a high-humidity air was blown tothe solution to obtain a honeycomb film. An upper surface of the thusprepared film was peeled off with the use of an adhesive tape to preparea pillar (protuberance) structure.

Ten (10) ml of MilliQ water were added dropwise to each sample and, in30 seconds later and 210 seconds later, the contact angle was measured.For the purpose of control, similar experimentation was carried outusing a plain film and a honeycomb pattern film, each of the samecomposition as mentioned above. By measurement, the contact angle withwater was 101°±9° for a plain film sample, 109°±2° for a honeycombsample, and 138°±2° for a pillar structure sample.

Each sample film was then immersed in a titanium alkoxide solution. Thefilm became slightly cloudy, because of light scattering on the surfaceof the film on which titania gels having a high refractive index wereformed by the hydrolysis of the titanium alkoxide. Even after rinsing,there was no change in the cloudy surface. In other words, the titaniagels would have been fixed to the film surface. The Cap polymer that wasthe amphiphilic polymer had a carboxyl group that would worked as ascaffolding for growth of titania gels on the film surface.

When the titania gels were formed on the film surface, the contact anglein 30 seconds after dropwise addition of water droplets thereto was39°±10° for a plain film sample, 19°±6° for a honeycomb sample, and31°±10° for a pillar structure sample, indicating that the samples weremade hydrophilic.

After the lapse of 210 seconds from the dropwise addition of waterdroplets, the contact angle became much smaller; that of the plain filmsample decreased to 28°±6°, and those of the honeycomb and pillarstructure samples decreased down to a level as small as could not detectany contact. The contact angle of the plain film was unlikely to becomelower than a constant value, whereas the contact angle of the structuredfilm became extraordinarily small, probably because the structuredhydrophilic surface gave rise to capillary force that caused the liquidto spread gradually over the surface of the film.

Wenzel has reported that generally as a surface area becomes large (thatis, a surface is provided with asperities), it works in such a way as toenhance the properties of the surface. An apparent contact angle θ_(w)is given bycos θ_(w)=r cos θHere r is the quotient of the area of a plain film divided by thesurface area of a structured film, and θ is the contact angle of theplain film. In other words, the larger the surface area, the moreenhanced the properties of the surface are. In this case, too, similarevents appear to have occurred.

Example 15 Impartation of Hydrophilicity by Ozone Treatment

A polystyrene (having a molecular weight of 280,000, Aldrich) and Capwere mixed together at a ratio of 10:1 to prepare 5.0 g/l of a solution.This solution (7.5 ml) was cast on a laboratory dish of about 9 cm indiameter, and a high-humidity air was blown to the solution to prepare afilm. Some upper surface of the film was peeled off with the use of anadhesive tape to prepare a pillar structure. Each of the obtained filmswas measured in terms of contact angle. At this time, the contact anglesof the honeycomb and pillar structure films were 114°±2° and 158°±5°,respectively.

Then, the prepared film was treated by an ozone cleaner (NL-UV253 madeby Nippon Laser Electronics Co., Ltd.), during which the contact angleon the film was measured every 30-minute ozone treatment. As a result,the contact angle was found to decrease slowly. However, the decrease inthe contact angle of the honeycomb film was gentle; it kept a contactangle of about 70° even after a 180-minute treatment, whereas the pillarstructure was more considerable than the honeycomb structure in terms ofthe decrease in the contact angle; the contact angle after the180-minute treatment was about 30°. This would indicate that the effectof the surface shape is more enhanced by the ozone treatment.

Example 16

A honeycomb film prepared from a poly(glycolic acid-lactic acid)copolymer having a weight-average molecular weight of 40,000 to 75,000(Aldrich) was sectioned off by peeling in a thickness direction with theuse of a tape to prepare a pillar structure. The surfaces of the pillarswere immersed in a 1N aqueous solution of sodium hydroxide for 120minutes, followed by washing with distilled water. After drying, thestatic contact angle with water droplets was measured after the lapse of30 seconds, and 120 seconds from the dropwise addition of waterdroplets. As a result, the film was found to have a contact angle of164°±2° before the immersion, but the contact angle of the film afterthe immersion (30 seconds and 120 seconds) was 21°±5° and about 0°,respectively.

Example 17

For surface coating, the surface of the pillar structure prepared inExample 16 was immersed in a 0.2 w/v % methanol solution of apoly(2-methoxyethyl acrylate) that was a water-insoluble, hydrophilicpolymer and had a weight-average molecular weight of 85,000. Afterdrying, the static contact angle with water droplets was measured afterthe lapse of 30 seconds, and 120 seconds from the dropwise addition ofwater droplets. As a result, the film was found to have a contact angleof 164°±2° before the coating, but the contact angle of the film afterthe coating (30 seconds and 120 seconds) was 29°±4° and about 0°,respectively.

From the above results, it was found that the honeycomb and pillarstructures can be made hydrophilic by surface chemical modification, andozone oxidization. Such films are expected to have possible applicationsto matrix materials, separation films or the like.

POSSIBLE UTILIZATION OF THE INVENTION IN THE INDUSTRY

According to the specific features of the invention that a dilutepolymer solution is cast on a solid substrate using water vapor as caststo obtain a thin film having a fine regular pattern of honeycombstructure and the thins film is bisected by peeling in a thicknessdirection, there can be provided quite an unheard-of material whereinmicro-pillars are regularly lined up and formed on the peeled sectionsof the thin film. This novel material, because of having micro-pillarsregularly lined up on its surface, could hereafter find applications,with a great deal of advantages, in the following various fields: aschemical valves, DNA chips, protein chips and cyto-diagnosis chips, forcell culture engineering, as medical scaffolding materials,semiconductors, recording materials, separators, ion exchange membranes,battery separator materials, optical materials for displays and lightguides, catalyst carriers, cell culture substrates and anisotropicsolid, electrically conductive materials, for micro-passageways, etc. Inparticular, the presence of micro-pillars makes it possible to provide asurface well fit for a biochip surface that controls material flows in aconstant direction, a low-friction-coefficient surface that reduces theresistance of air or water in a constant direction alone or the like.Such embodiments, if anisotropy is imparted to micro-pillars to addfurther enhancements to their own advantages, could find wideapplications in various fields and make a great deal of contribution todevelopments in the industry.

1. A process for preparing a micro-pillar structure, characterized inthat a solution having a polymer dissolved in a hydrophobic organicsolvent is cast on a substrate, said organic solvent is evaporated in amoist atmosphere of relative humidity of 50% or higher to condensemoisture contained in an atmosphere prevailing on a surface of said castsolution into micro-droplets, said micro-droplets are dispersed on thesurface of said cast solution or in said cast solution into a packedstructure, said micro-droplets, condensed and dispersed on the surfaceof said cast solution or in said cast solution, are evaporated to obtaina porous honeycomb structure with said droplets used as casts, and saidporous honeycomb structure is at least bisected by peeling in athickness direction, thereby obtaining honeycomb structures whereinmicro-pillars are regularly formed and arranged by said bisection onpeeled sections, characterized in that said polymer is composed of ahydrophobic or biodegradable polymer and an amphipathic polymer and,optionally, a modifier is incorporated therein, and said hydrophobicpolymer comprises a polymer having a polystyrene basic skeleton and saidbiodegradable polymer comprises a polymer having a poly(meth)acrylatebasic skeleton.
 2. A process for preparing a micro-pillar structureaccording to claim 1, wherein said polymer comprises 50 to 99% of saidhydrophobic polymer or said biodegradable polymer with the rest being anamphiphilic polymer.
 3. A process for preparing a micro-pillar structureaccording to claim 1, wherein said moist atmosphere is adjusted to arelative humidity of 50 to 95%.
 4. A process for preparing amicro-pillar structure according to claim 1 or 3, characterized in thatsaid atmosphere is an ordinary air atmosphere.
 5. A process forpreparing a micro-pillar structure according to claim 1, characterizedin that operation for evaporation of said organic solvent in said moistatmosphere is carried out by blowing an atmosphere having a highhumidity onto an evaporation interface of said organic solvent.
 6. Aprocess for preparing a micro-pillar structure according to claim 1,characterized in that peeling operation is carried out by use of anadhesive tape.
 7. A process for preparing a micro-pillar structureaccording to any one of claims 1, 3, 5 and 6, characterized in that saidmicro-pillars are arranged at a length of 0.1 to 50 μm, a tip length of0.01 to 20 μm and a spacing of 0.1 to 100 μm.
 8. A process for preparinga micro-pillar structure according to any one of claims 1, 3, 5 and 6,above, wherein said micro-pillars are oriented in an direction exceptfor a vertical direction and set with anisotropy.
 9. A process forpreparing a micro-pillar structure according to claim 8, characterizedin that said anisotropic micro-pillars are obtained by a peelingtreatment with transverse shearing stress in such a way that when theporous honeycomb structure that is a micro-pillar precursor is sectionedby peeling in the thickness direction, the resulting micro-pillars areoriented in any direction except for the vertical direction.